This invention relates to multilayer plated aluminum sheets suitable for use as automobile body panels. More particularly, it is concerned with aluminum sheets which have two or more plated coating layers, which can be subjected to zinc phosphating without substantial dissolution of A1 ions, and which exhibit good corrosion resistance before and after finish paint coating as well as good plating adhesion and good press formability, all of these properties being required for automobile body panels.
In recent years, the requirements for steel sheets used as automobile body panels have become increasingly severe with respect to protection from corrosion caused by antifreezing agents spread on roads in cold regions and with respect to weight reduction for decreasing fuel consumption. In order to protect automobile bodies from corrosion, surface-treated steel sheets such as zinc- or zinc alloy-plated steel sheets have been applied to such panels. As a measure for weight reduction, use of high tensile strength steel sheets has increased since the sheet thickness can be reduced with such steel sheets.
In order to further reduce the weight of automobile bodies, in more recent years, aluminum sheets made of aluminum or an aluminum alloy have been used in some automobile bodies. In such cases, aluminum sheets usually constitute only part of the body panels required to assemble an automobile body, the remaining portion of the body panels being comprised of steel sheets. Thus, in most cases, aluminum sheets are used together with steel sheets to assemble automobile bodies.
A typical assembly line for automobile bodies made of steel sheets includes the steps of press-forming steel sheet panels into desired shapes, assembling the formed steel sheets by means of resistance spot welding to form an automobile body, and finally subjecting the assembled body to zinc phosphating, electro-deposition coating, and spray coating in that order. When aluminum sheets are partly applied to automobile bodies, it is desired that it be possible to process aluminum sheets along with steel sheets in the same assembly line of the above-mentioned sequence. This eliminates the necessity to install a separate assembly line for aluminum sheets and maintains the continuity of the assembly process.
However, in such cases, a problem occurs in the zinc phosphating stage. Zinc phosphate treatment is applied prior to painting in order to improve the adhesion of paint coating and hence the corrosion resistance of automobile bodies.
Zinc phosphating of aluminum sheets, however, not only does not form a good zinc phosphate coating on the surface of each aluminum sheet, but also causes dissolution of the aluminum sheet at the surface thereof, thereby contaminating the zinc phosphating solution with A1 ions dissolved out of the sheet. As a result, as the concentration of A1 ions in the zinc phosphating solution is increased to as low as several parts per million, steel sheets treated in the solution are adversely affected such that a good zinc phosphate coating can no longer be formed on the steel surfaces.
In order to solve this problem, it is proposed in Japanese Unexamined Patent Publication No. 61-157693(1986) that the coatability of an aluminum sheet with zinc phosphate can be improved by forming a plated coating of zinc, a zinc alloy, or an iron alloy at a weight of at least 1 g/m.sup.2 on the surface of the sheet. According to that publication, since the plated coating protects the aluminum sheets sufficiently to prevent dissolution of A1 ions in a zinc phosphating solution during subsequent zinc phosphating stage, a satisfactory zinc phosphate coating can be formed on both of aluminum and steel surfaces when aluminum sheets and steel sheets are treated in the same solution.
However, the plated coating is formed by electroplating in an acidic sulfate bath prior to press forming according to the method described in that publication. It is well known that the surface of an aluminum sheet is covered with a thin oxide film, which greatly interferes with deposition of electroplated coating. Therefore, the resulting plated coating has poor adhesion and it may readily peel off from the aluminum sheet during press forming, thereby making it difficult to achieve the above-described desired effects of the plated coating.
In order to overcome this problem, it is proposed in Japanese Unexamined Patent Publication No. 3-146693(1991) that the surface of an aluminum sheet be coated with a first layer of a nickel-plated coating, which is formed either by displacement plating in an acidic chloride bath containing hydrogen fluoride or by electroplating in an acidic sulfate bath. A zinc- or zinc alloy-plated coating is formed on the first nickel layer, and it has good adhesion to the underlying first layer.
However, since Ni is nobler than A1, A1 has a higher ionization tendency, and the first nickel layer may cause galvanic corrosion of the base aluminum sheet, which, in turn, may cause blistering of the overlaid zinc- or zinc alloy-plated layer and finish paint coating, thereby significantly degrading corrosion resistance in those areas where the finish paint coating is injured. Furthermore, the nickel layer is stiff and susceptible to flaking. When the resulting flake penetrates into the aluminum sheet, a notch is formed and it may cause the aluminum sheet to be broken during press forming by stress concentration at the notch.
Aluminum sheets have high surface activity and form on the surface thereof a firm oxide film which is readily regenerated after removal. Such an oxide film significantly interferes with the adhesion of a plated coating formed thereon. Therefore, when an aluminum sheet is electroplated, the sheet is usually pretreated so as to remove the oxide film immediately before electroplating.
For this purpose, displacement plating (also called immersion plating) with zinc or a zinc alloy is employed. The displacement plating with zinc or a zinc alloy is merely intended to remove the oxide film and enhance the adhesion of an electroplated coating formed thereon.
Japanese Unexamined Patent Publications Nos. 2-19488(1990), 2-19489(1990), and 2-19490(1990) disclose pretreatment of an aluminum sheet with a zincate bath to form a first zinc coating by displacement plating before the sheet is electroplated with a zinc alloy in a basic bath. However, the use of a basic bath in the electroplating stage may result in pitting corrosion and alkaline dissolution of the aluminum sheet caused by hydroxyl ions present in the bath, thereby re-exposing the surface of the aluminum sheet and degrading the adhesion of the zinc alloy plated coating formed thereon.
It is also well known that spot welding of aluminum sheets is more difficult than spot welding of steel sheets since aluminum is higher than steel in both electric and thermal conductivity. The spot weldability of aluminum sheets is particularly poor with respect to the number of maximum weldable spots in continuous spot welding before the electrodes of a spot welder are damaged (hereafter referred to merely as "number of weldable spots"). For example, more than 3,000 spots can be continuously welded with steel sheets before the electrodes of the spot welder are damaged. In contrast, in spot welding of aluminum sheets, the number of weldable spots is on the order of 300 to 500, and the spot welding line must be stopped after welding of 300 to 500 spots to exchange or grind the damaged electrodes, thereby significantly decreasing the efficiency.
Electroplating of aluminum sheets with zinc or a zinc alloy, for example, by the method disclosed in Japanese Unexamined Patent Publication No. 3-146693(1991) also serves to improve the spot weldability of the sheets. However, there is a need of further improvement in spot weldability of aluminum sheets.